The present invention relates in general to electronic oscillators and in particular to emitter coupled oscillators of the type wherein the frequency and duty cycle of the oscillator are adjustable.
A typical emitter coupled oscillator comprises a matching pair of transistors having emitters coupled by a capacitor. The base of each transistor is coupled in a feedback loop to the collector of the other. An adjustable current source is applied directly to the emitter of one of the transistors and a second adjustable current source is applied through a switch to the second transistor emitter.
Oscillator output is taken at the base of either transistor and oscillation is initiated by switching the second current source from ground to the second transistor emitter. The frequency of oscillation may be increased or reduced by increasing or reducing the current supplied by the current sources. The duty cycle may be altered by changing the amount of current supplied by one current source relative to the other.
Problems are associated with this circuit. First, after switching the second current supply to the emitter of the second transistor, there may be a delay before the oscillator makes its first change of state. The length of the delay depends on the charge on the capacitor at the time the second current supply was switched on. If that charge is different from the charge on the capacitor at the moment the oscillator changes state during steady state oscillation, then there is a delay in switching until the capacitor charge reaches the proper level. The initial delay in oscillator change of state also depends on the magnitude of the second current source. Since the current source may be adjusted to program the oscillator for different operating frequencies and duty cycles, the initial delay in oscillator change of state can vary, making it difficult to synchronize oscillator operation with external events.
Secondly, because the charge on the capacitor at the time the oscillator is switched on may be different from the charge on the capacitor at the moment of oscillator change of state during steady state operation, the cycle times associated with the first few oscillations may be longer or shorter than the steady state cycle time of the oscillator. For instance, if the capacitor has a high initial charge, the first period of oscillation will be long, with subsequent periods being progressively shorter as inherent losses in the oscillator dissipate the excess energy associated with the excess charge until the steady state period is reached.
The traditional emitter coupled oscillator is unable to maintain proper output voltage levels at low frequencies, the frequency of operation and the output voltage levels of the oscillator being dependent on the magnitudes of the current sources. When the current sources are adjusted to a low current output to obtain low frequency oscillator operation, the low current values are not sufficient to support the low logic output levels. Thus the frequency range of the oscillator is limited.
Additionally, at the moment the oscillator is turned off by disconnecting the second current source, the emitter coupling capacitor may be charged to a value other than its steady state, "oscillator off" value and the output voltage signal may incur some transient ringing as the charge on the capacitor settles to its steady state magnitude.
Finally, the last period of the oscillator may be abruptly and prematurely terminated when the oscillator is turned off. In some applications it is desirable that oscillation cease only at the end of a full period of oscillation so that the last period is of the same duration as the first.
What is needed is an emitter coupled oscillator having a minimal and consistent time delay for the first change of state and a minimal transient response when turned on. The oscillator should also have a wide operating frequency range and the ability to stop with minimal transient response at a predetermined point in the oscillation cycle when turned off.